This invention relates generally to the cryogenic rectification of feed air and, more particularly, to the cryogenic rectification of feed air to produce low purity oxygen.
The demand for low purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Low purity oxygen is generally produced in large quantities by the cryogenic rectification of feed air. However, conventional cryogenic rectification systems for producing low purity oxygen are relatively inefficient.
Accordingly it is an object of this invention to provide a cryogenic rectification system which can more efficiently produce low purity oxygen.
The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing low purity oxygen comprising:
(A) passing feed air into a higher pressure column and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid;
(B) passing nitrogen-enriched fluid and oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid by cryogenic rectification within the lower pressure column;
(C) passing oxygen-richer fluid into the upper portion of an auxiliary column and producing low purity oxygen fluid in the lower portion of the auxiliary column;
(D) withdrawing reboiling fluid from an intermediate level of the higher pressure column, increasing the pressure of the withdrawn reboiling fluid by cold compression, and vaporizing a portion of the low purity oxygen fluid by indirect heat exchange with the increased pressure withdrawn reboiling fluid; and
(E) recovering low purity oxygen fluid from the lower portion of the auxiliary column as product low purity oxygen.
Another aspect of the invention is:
Apparatus for producing low purity oxygen comprising:
(A) a higher pressure column and means for passing feed air into the higher pressure column;
(B) a lower pressure column and means for passing fluid from the higher pressure column into the lower pressure column;
(C) an auxiliary column having a bottom reboiler, and means for passing fluid from the lower portion of the lower pressure column into the upper portion of the auxiliary column;
(D) a cold compressor, means for passing fluid from an intermediate level of the higher pressure column to the cold compressor, and means for passing fluid from the cold compressor to the auxiliary column bottom reboiler; and
(E) means for recovering product low purity oxygen from the lower portion of the auxiliary column.
A further aspect of the invention is:
A method for producing low purity oxygen comprising:
(A) cooling feed air in a main heat exchanger to produce cooled feed air, passing a portion of the cooled feed air into a higher pressure column, and separating the feed air within the higher pressure column by cryogenic rectification into nitrogen-enriched fluid and oxygen-enriched fluid;
(B) passing nitrogen-enriched fluid and oxygen-enriched fluid from the higher pressure column into a lower pressure column and producing oxygen-richer fluid by cryogenic rectification within the lower pressure column;
(C) passing oxygen-richer fluid into the upper portion of an auxiliary column and producing low purity oxygen fluid in the lower portion of the auxiliary column;
(D) increasing the pressure of another portion of the cooled feed air by cold compression, and vaporizing a portion of the low purity oxygen fluid by indirect heat exchange with the increased pressure feed air portion; and
(E) recovering low purity oxygen fluid from the lower portion of the auxiliary column as product low purity oxygen.
Yet another aspect of the invention is:
Apparatus for producing low purity oxygen comprising:
(A) a main heat exchanger, a higher pressure column, means for passing feed air to the main heat exchanger, and means for passing feed air from the main heat exchanger to the higher pressure column;
(B) a lower pressure column and means for passing fluid from the higher pressure column into the lower pressure column;
(C) an auxiliary column having a bottom reboiler, and means for passing fluid from the lower portion of the lower pressure column into the upper portion of the auxiliary column;
(D) a cold compressor, means for passing feed air from the main heat exchanger to the cold compressor, and means for passing feed air from the cold compressor to the auxiliary column bottom reboiler; and
(E) means for recovering product low purity oxygen from the lower portion of the auxiliary column.
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein, the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
The term xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein, the term xe2x80x9ctopxe2x80x9d when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing.
As used herein, the term xe2x80x9cbottomxe2x80x9d when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing.
As used herein, the term xe2x80x9creboilerxe2x80x9d means a heat exchange device that generates column upflow vapor from column liquid. A reboiler may be located within or outside of the column. A bottom reboiler generates column upflow vapor from liquid from the bottom of a column. An intermediate reboiler generates column upflow vapor from liquid from above the bottom of a column.
As used herein, the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the midpoint of the column.
As used herein, the term xe2x80x9ctrayxe2x80x9d means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray.
As used herein, the term xe2x80x9cequilibrium stagexe2x80x9d means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP).
As used herein, the term xe2x80x9clow purity oxygenxe2x80x9d means a fluid having an oxygen concentration within the range of from 70 to 98 mole percent.
As used herein, the term xe2x80x9ccold compressorxe2x80x9d means a device for raising the pressure of a vapor in which both the inlet and discharge streams are below ambient temperature.
As used herein, the term xe2x80x9ccold compressionxe2x80x9d means a process using a cold compressor for raising the pressure of a subambient temperature vapor stream requiring energy input.